Many systems exist for extracting useful forms of energy from renewable sources, such as waves, tides, wind and the like. For example, it is known in the art to convert ocean wave power to a useful form using flexible or elastic bodies, including tubes, balloons, membranes and the like. These systems are typically arranged such that the hydrodynamic pressures exerted by water waves result in changes in pressure within a contained fluid or gas. These captive pressure variations are then subsequently converted to mechanical power via various mechanisms (e.g. turbines, pistons and the like) with mechanical power then being converted to electricity or high pressure water by means of a generator, pump or similar.
GB 2 434 840, for example, discloses such a device in the form of a tubular wave energy converter. Oscillating pressure of seawater outside the tube wall due to ocean waves creates a localised pressure gradient across the tube wall relative to the sea water inside the tube. The walls of the tube have a high elasticity so that they can distend in response to the pressure gradient, so inducing a bulge wave within the tube. The bulge wave has a natural propagation speed within the tube dictated by the tube's distensibility. If the propagation speed of the bulge and ocean waves match, resonant energy transfer takes place as the bulge wave “surfs” the exciting ocean wave. Thus, the bulge wave progressively sucks energy from the ocean wave resulting in a progressive increase in amplitude of the bulge wave. The distensibility of the tube is fixed and designed in order to match the bulge wave propagation velocity with the predominant expected ocean wave velocity, thus maximising average wave energy absorption. The energy accumulated in the bulge wave is eventually converted into useable form via a power take-off apparatus or process located at one or both ends of the tube.
However, in such known distensible tube arrangements, the distensibility of the tube and hence the speed of the bulge wave are fixed. Thus, the tube will respond well to some frequencies of wave but not so well to others. In reality ocean waves have widely varying frequencies and hence the tube will not be matched to incident conditions. This will limit the overall average energy abstraction.
Additionally, it is known that the bulge wave grows as it propagates along the tube and energy is accumulated along the length of the tube. Accordingly, a thicker tube wall is necessary to accommodate the stored energy, with attendant additional weight, manufacturing and handling difficulties and cost. There may be distortion of the bulge wave due to gross deformation of the tube.
Furthermore, the bulge wave may saturate before it reaches the end of the tube. Thus, a portion of the length of the tube may be redundant and does not contribute to energy generation.
Additionally, features of the device, such as the power take off apparatus, may not be matched to the dynamic response of the tube, and as such reflections may occur. These reflections may serve to lower or limit the size and energy stored by the bulge wave through destructive interference, reducing the energy capture of the device.
There are a multitude of wave energy converters that achieve effective absorption of energy. Conversion to useful power, however, remains a significant challenge which has not yet been satisfactorily met by the art. Numerous machines and processes exist but these all suffer from reliability problems inherent in electro-mechanical equipment exposed to a harsh environment such as the sea. In addition, the majority of known power take-off systems do not permit good efficiencies to be achieved.